Engine/Transmission (1998.5 - 2002) Lift pump questions...

[Dane]

Can someone explain to me why the ECM pulses the lift pump?



And what parameters does the ECM use to decide when to pulse the pump?



Thanks!

 

[A Johnson]

During normal operation lift pump voltage does not pulse. It is 100% duty cycle to provide 15 PSI.



During engine cranking it pulses on and off to limit lift pump pressure to below 5 PSI. The reason for this is so that lift pump pressure during cranking will be below VP-44 internal vane pump pressure to facilitate easier starting.



What happpens to the best of my understanding is lift pump pressure will "back feed" inside the VP-44 and hold the plungers of the high pressure pump off the internal cam ring, resulting in no injection pressure, until internal vane pump pressure exceeds vane pump presssure. I have had several VP-44s in pieces and I have not been able to tell where the back feed flow goes exactly, so this is the best explanation I'm able to give at this time.



A Johnson

 

[Gary - K7GLD]

AJ - since you have had a couple of VP-44's apart, what was their point of failure - and what is you over-all opinion of the quality and potential durability of internal components - outside of their obvious failure points?

 

[A Johnson]

Gary, On the first pump the timing piston was scored/ stuck giving Cummins fault 368. This can be caused by lack of cooling/ lubrication, dirt/water contamination etc. I believe the bore for the timing piston was changed to a brass type material from aluminum, to help solve this seizure problem.



Second pump was a no start, and it had the internal pulsation dampener broken which, (I have to be careful how I word this as I don't want to be taken to task for not giving accurate info) caused fuel to keep from building pressure in the high pressure pumping area. The dampener is also supposed to absorb high pressure spikes from the ending of the injection cycle. ( I'm not sure if these are the wild pressure spikes some are seeing in the low pressure fuel system or not) Anyway this dampener has also had a material change for improved durability.



Pump number three was replaced at the request of a customer, and I never saw a problem with it. Just tore it down to have a look see.



Now for my opinions: 1- I think Bosch and Cummins have identified and modified a number of internal VP issues as well as VP electronic issues and a reman or new pump that you buy today is better than what came on the rig. (other than late model 2002)



2- The internal vane pump that produces 300psi internal pressure, has vanes that slide along an aluminum housing and I think there is potential for galling. This along with the timing plunger sticking issue is where I think adequate lift pump pressure is of great importance. ( Gary I think you have done an excellent job on your system)



3- The injection control pin is very small and I don't understand how it can stand up, doing 6000 injection cycles per minute. (Isn't that the number if a six cylinder is doing 2000rpm?)



4- If Cummins had to do the ISB all over I think it would have a mechanical lift pump with perhaps an electronic governor for low pressure fuel supply. At least something other than the lift pump they have today.







5- I would buy a 24 valve with VP-44 today and modify the low pressure system to my needs.



If I need to clarify anything more just let me know.



A Johnson